The new wolf variants aren't spawning nearly as much as they should be. I went to the Old growth pine taiga in my world that was created in version 1.18 and no black dogs were spawning after standing there for several in-game days. I created a brand new world in version 1.20.5 and black dogs spawned almost immediately when the chunks loaded in.  I'm thinking there may be a problem with the spawn rates in relation to already-created worlds.